184
Ferran Rodil, Xavier Mayor, Santiago Sabate and Victoria Diego
learnt in the follow-up of the experiment during the fourth year, which was
exceptional in having a high spring-summer rainfall.
13.2 Experiment Layout
Th~ experiment was performed in the Prades experimental area (NE Spain;
Chap. 2), in a stand of closed-canopy holm oak, which shows the typically
high density of small-sized stems developed by resprouting after earlier coppicing. This stand structure is highly relevant to the present experiment,
since the ability of individual stems to respond to added water or nutrients
might be limited in a crowded canopy by the low amounts of light intercepted by their narrow crowns.
The stand occupied a mid-slope position on a steep, southeast-facing slope
at an altitude of 900 m. The soil was a shallow, stony, clay-loam Xerochrept
on phyllites and metamorphic sandstones. The tree layer was 6 m high. At
the start of the experiment, the density of stems of all species having a diameter at 50 cm from the ground of ~ 2 cm averaged 18275 stems ha- I . Mean
basal area at 50 cm from the ground was 47.9 m 2 ha- I . Aboveground biomass
of the tree layer averaged 128 Mg ha- I (range 81-177 Mg ha- I ), a typical figure for closed-canopy holm oak forests in NE Spain (Chap. 3). Holm oak accounted on average for 91 % of total aboveground biomass.
Details of the experimental design and methods can be found in Mayor
and Roda (1994) and Sabate and Gracia (1994). Briefly, we used a factorial
design with eight treatments combining three factors (irrigation, N fertilization, and P fertilization), each at two levels: with and without experimental
addition. We used three replicated plots per treatment. Weekly drip irrigation
(ca. 20 mm week-I) was applied during the warm seasons of 1989 through
1991. Fertilization was conducted in March 1989 in a single dose. N-fertilized
plots received 250 kg N ha- I as ammonium nitrate. P-fertilized plots received
125 kg P ha- I as calcium superphosphate.
13.3 Aboveground Net Primary Production
Aboveground net primary production (ANPP) was estimated as the sum of
aboveground biomass increment and fine litterfall. Stem diameters were measured before the start of the experiment, and again 3 years later. These data and
allometric regressions derived for this forest (Lled6 1990; S. Sabate et al. unpubl. results) allowed us to compute the aboveground biomass. Fine litterfall
was collected monthly. Here, we focus on the main results obtained in the first 3
years after treatment application began (1989-1991). Overall, this was a dry period, and rainfall was below average during the springs of 1989 and 1990.
Ferran Rodil, Xavier Mayor, Santiago Sabate and Victoria Diego
learnt in the follow-up of the experiment during the fourth year, which was
exceptional in having a high spring-summer rainfall.
13.2 Experiment Layout
Th~ experiment was performed in the Prades experimental area (NE Spain;
Chap. 2), in a stand of closed-canopy holm oak, which shows the typically
high density of small-sized stems developed by resprouting after earlier coppicing. This stand structure is highly relevant to the present experiment,
since the ability of individual stems to respond to added water or nutrients
might be limited in a crowded canopy by the low amounts of light intercepted by their narrow crowns.
The stand occupied a mid-slope position on a steep, southeast-facing slope
at an altitude of 900 m. The soil was a shallow, stony, clay-loam Xerochrept
on phyllites and metamorphic sandstones. The tree layer was 6 m high. At
the start of the experiment, the density of stems of all species having a diameter at 50 cm from the ground of ~ 2 cm averaged 18275 stems ha- I . Mean
basal area at 50 cm from the ground was 47.9 m 2 ha- I . Aboveground biomass
of the tree layer averaged 128 Mg ha- I (range 81-177 Mg ha- I ), a typical figure for closed-canopy holm oak forests in NE Spain (Chap. 3). Holm oak accounted on average for 91 % of total aboveground biomass.
Details of the experimental design and methods can be found in Mayor
and Roda (1994) and Sabate and Gracia (1994). Briefly, we used a factorial
design with eight treatments combining three factors (irrigation, N fertilization, and P fertilization), each at two levels: with and without experimental
addition. We used three replicated plots per treatment. Weekly drip irrigation
(ca. 20 mm week-I) was applied during the warm seasons of 1989 through
1991. Fertilization was conducted in March 1989 in a single dose. N-fertilized
plots received 250 kg N ha- I as ammonium nitrate. P-fertilized plots received
125 kg P ha- I as calcium superphosphate.
13.3 Aboveground Net Primary Production
Aboveground net primary production (ANPP) was estimated as the sum of
aboveground biomass increment and fine litterfall. Stem diameters were measured before the start of the experiment, and again 3 years later. These data and
allometric regressions derived for this forest (Lled6 1990; S. Sabate et al. unpubl. results) allowed us to compute the aboveground biomass. Fine litterfall
was collected monthly. Here, we focus on the main results obtained in the first 3
years after treatment application began (1989-1991). Overall, this was a dry period, and rainfall was below average during the springs of 1989 and 1990.